In recent years, there has been a rapid reduction in the size and weight of mobile information terminals such as mobile telephones, notebook personal computers, and PDA. Higher capacity is required of cells and batteries serving as the driving power sources of such terminals. Non-aqueous electrolyte secondary cells represented by lithium ion secondary cells have high energy density and high capacity and as such are widely used as the driving power sources of the mobile information terminals.
Generally, non-aqueous electrolyte secondary cells use a positive electrode made of a lithium-containing transition metal compound oxide, a negative electrode made of carbon material such as graphite, and a non-aqueous electrolyte containing a lithium salt dissolved in a non-aqueous solvent. In such cells, there is migration of lithium ions between the positive electrode and the negative electrode during charge and discharge, and internal short circuiting caused by dendrite lithium does not occur because no lithium exists in the state of metal. Such cells therefore excel in safety.
However, such non-aqueous electrolyte secondary cells can be problematic in that overcharge causes an excessive release of lithium ions from the positive electrode and an excessive storage of the lithium ions in the negative electrode. This lowers the thermal stability of both electrodes, deteriorating cell characteristics. Also, an extremely unbalanced differential between the electrodes decomposes the electrolytic solution. The decomposition of the electrolyte causes, as well as gas generation, heat generation resulting from an increase in internal cell resistance. As a result, there can be a rapid increase in internal cell pressure, causing cell burst and thermal runaway.
In view of these problems, the non-aqueous electrolyte secondary cells have incorporated therein a current-cutting device for cutting an overcharged current, upon generation of such a current. However, the current-cutting device operates to cut the current only upon increase in internal cell pressure, and there is a time-lag between abnormality to occur in the cell and the increase of internal cell pressure. Thus, it takes a long time before the current-cutting device operates, and there is a doubt as to ensuring security in the case of an intense temperature increase.
Also in view of the problems, there have been proposed techniques of adding various additives in the non-aqueous electrolyte. For instance, Japanese Unexamined Patent Publication No. H5-36439 discloses a technique of adding a linear alkylbenzene derivative in a non-aqueous solvent in a non-aqueous electrolyte secondary cell having a current-cutting device. With this technique, at the time of overcharge, the linear alkylbenzene derivative is dissolved to generate methanes that in turn consume an active oxygen detached from the positive electrode by reacting with the oxygen. Thus, this technique is aimed at preventing a temperature increase caused by the active oxygen. However, since the linear alkylbenzene derivative operates neither to cut an overcharged current nor to increase the response rate of the current-cutting device, the technique cannot ensure security in the case of an intense temperature increase.
Japanese Unexamined Patent Publication No. H9-106835 discloses a technique of preventing overcharge by using a non-aqueous solvent having added therein thiophene, biphenyl, furan, and the like. With this technique, the compounds thiophene, biphenyl, and furan polymerize at a potential higher than or equal to the highest cell operation voltage and form a highly resistive film on the electrode surfaces, thus trying to prevent overcharge. However, this technique is problematic in that the above compounds cause to lower power generation performance, and that since the compounds polymerize only under a high temperature of 120° C. or higher the current cutting-off cannot be realized with the use of the compounds unless the cell temperature becomes high.
On the other hand, the present inventors suggested in Japanese Unexamined Patent Publication No. 2001-15155 a technique of preventing overcharge by adding in a non-aqueous solvent a cycloalkylbenzene derivative or an alkylbenzene derivative having a tertiary carbon adjoining a phenyl group. These additives, suggested by the present inventors, are chemically decomposed at the time of overcharge and generate a hydrogen gas, and the molecules polymerize together to form a film on the negative electrode surface. This film is stable and insoluble in the non-aqueous solvent and has high electrical resistance. With this technique, the hydrogen gas generated from the electrode and the highly resistive film operate to rapidly increase internal resistance and prevent overcharge, thereby ensuring security in the case of an intense temperature increase.
When the non-aqueous solvent having added therein the above additives is used in a non-aqueous electrolyte secondary cell provided with the current-cutting device, the following advantageous effect is obtained in addition to the above effect. The hydrogen gas generated from the electrode increases internal cell pressure, thereby increasing the response rate of the current-cutting device. It should be noted, however, that these compounds are decomposed to form a highly resistive film on the active material surface of the negative electrode if the cell is used under a high temperature environment of 40° C. to 60° C., regardless of the overcharged state. Thus, when using the cell under a high temperature environment, there is the problem of lowering cell performance such as cell cycle characteristics.